Method of surface treating high-strength aluminum

ABSTRACT

The disclosure relates to a method of coating a high-strength aluminum object with polymer and surface-treating it, for improved corrosion resistance. A polymer composition is coated onto the surface of the aluminum object and is sintered or melted fast, at the same time as solution treatment for precipitation hardening takes place. The polymer composition substantially comprises a fluorine-containing polymer, preferably PTFE. According to one preferred embodiment of the invention, the polymer coating is sintered or melted fast on the aluminum surface during a time period of approx. 15 minutes at approx. 420 degrees C. After solution treatment and simultaneous surface treatment at elevated temperature, the aluminum object is rapidly cooled to room temperature and precipitation hardened thereafter by means of artificial aging preferably at approx. 120-150 degrees C. for approx. 24 hours.

TECHNICAL FIELD

The present invention relates to a method of coating with polymer andsurface treating an object of high-strength aluminium.

BACKGROUND ART

Objects of high-strength aluminium are often used as structuralmaterials for machine parts on which demands are placed for light weightand high strength, for example in aircraft structures.

High-strength aluminium is obtained by precipitation hardening (orso-called age hardening) of a so-called heat treatable aluminium alloyby a two-stage heat treatment process. In the first stage, during theso-called precipitation treatment, the material is heated to an elevatedtemperature at which all alloy components are dissolved in the crystallattice structure of the aluminium and are transformed into so-calledsolid solution. The greater the proportion of alloy components which thealloy contains, the higher will be the temperature required forsolution. The solution treatment is terminated in that the object israpidly cooled with water, water mist or air. In the second stage,during the so-called ageing process, hardening precipitations are formedin the material. Ageing of high-strength aluminium takes place atelevated temperature for a relatively short time, so-called artificialageing, as opposed to so-called cold ageing, i.e. ageing at roomtemperature over a relatively lengthy period of time.

Aluminium material is generally highly resistant to corrosion in aneutral environment because of the fact that the aluminium surface isoxidised and the thus formed oxide layer is relativelycorrosion-resistant. In acidic (pH<4) and alkaline (pH>9) environments,this oxide layer becomes, however, unstable and so the materialcorrodes.

In order to achieve improved corrosion resistance, machine parts andstructures for use in acidic or alkaline environments can be surfacetreated by means of coating with a suitable chemical-resistant polymerpossessing superior internal strength and adhesion to the surface of thealuminium object, such as, for example, polymers containing fluorine.Fluorine-containing polymers normally also possess superior thermalresistance, which is an advantage in many fields of practicalapplication.

One particular field of application for such polymer-coatedcorrosion-resistant aluminium objects is machine pats in fillingmachines intended for the packing of liquid foods of the type whichfills, forms and seals packages in the same machine. In the handling offoods, extremely high demands are placed on hygiene and cleanliness,these demands being satisfied in that those parts of the machines whichare in direct content with the food are regularly cleaned (i.e. at leastonce a day) by means of efficient detergents or cleaning agents. Suchcleaning agents often contain alkaline chemicals. In cleaning, it isinevitable that detergents and cleaning liquids splash and drop ontoother parts of the machine. In particular, machine parts which areincluded in the sealing unit such as, for example, sealing jaws, areoften located beneath the filler pipe and the conduits which lead to andfrom the filler unit, which, on cleaning of the filler unit, inevitablyresults in cleaning agent dripping down onto these machine parts.

Surface treatment of high-strength aluminium by means of polymer coatingtoday is put into effect in that the finished, already precipitationhardened and ready-to-use aluminium object is coated with a layer ofpolymer and then heated to elevated temperature in order to sinter ormelt the polymer coating fast to the aluminium surface. How high thetemperature which is to be selected is a matter of discretion takinginto account the properties of the polymer and the temperatureresistance of the aluminium. The term sintering (also known asagglomeration) is taken to signify the physical process which takesplace when more or less solid material particles bond or frit to oneanother by molecular diffusion in the surface layer and thus “migratetogether” to form a continuous microporous network.

Commercially available polymer compositions with a high melting pointand which are sintered at high temperatures such as, for example,approx. 400° C. display generally better adhesion, mechanical propertiesand resistance to chemicals. Heating to such elevated temperaturesentails, however, that the aluminium material loses both hardness andmechanical strength by more than 50% up to approx. 65-75%. In practice,polymers are therefore employed which melt and sinter at lowertemperatures, such as, for example, at approx. 200° C. Coatings of suchpolymers unfortunately display poorer adhesion to the aluminium surfaceand, as a result, afford a poorer corrosion protection, while, on theother hand, the hardness and mechanical strength of the aluminium objectare retained on heating up to at most approx. 200° C.

Even though such plastic-coated machine parts of high-strength aluminiumtoday constitute the most corrosion-resistant alternatives on the marketwhich also satisfy other design and construction requirements, they mustbe replaced after a relatively short service life because the polymercoating has been attacked and weakened by the alkaline substances and nolonger affords blanket protection for the aluminium object which, as aresult, will be destroyed by corrosion. It is, thus, an as yet unsolvedproblem within the prior art technology to surface-coat objects ofhigh-strength aluminium in order to achieve improved corrosionresistance to a sufficiently high degree without negatively influencingthe mechanical strength and durability properties of the aluminiumobject.

OBJECTS OF THE INVENTION

One object of the present invention is, therefore, to realise a novelmethod of surface-treating objects of high-strength aluminium asdescribed by way of introduction, without consequential problems of thetype inherent in the prior art technology.

A further object of the present invention is to realise a method ofproducing surface-treated objects of high-strength aluminium withimproved corrosion resistance.

A particular object of the present invention is to realise a method ofproducing objects of high-strength aluminium possessing improvedcorrosion resistance and retained pristine high strength and superiormechanical properties.

Still a further object of the present invention is to realise acorrosion-resistant object of high-strength aluminium which issurface-treated with polymer and is produced using the method accordingto the present invention.

SOLUTION

These and other objects will be attained according to the presentinvention by the method which carries the characterizing features as setforth in the characterizing clause of appended claim 1. Variations andmodifications of the method according to the present invention areapparent from appended subclaims 2 to 12.

Further, the present invention realises surface-treated objects ofhigh-strength aluminium according to appended claim 13, with improvedcorrosion resistance and retained pristine mechanical properties.

OUTLINE OF THE INVENTION

In so-called heat-treatable aluminium alloys, one or more of the alloycomponents are selected such that a strength increase is achieved byprecipitation hardening, also called age hardening. The precondition forprecipitation hardening to be able to take place is that the solubilityof the added alloy components in aluminium reduces with fallingtemperature. Thus, precipitation hardening is achieved by solutiontreating, in a first stage, i.e. for a relatively short time heating theheat-treatable aluminium alloy to such an elevated temperature that theadded alloy components merge into solid solution within the aluminiumstructure, and subsequently rapidly cooling the alloy so that asaturated solution of alloy atoms in the aluminium material remains and,thereafter, in a second stage ageing the aluminium alloy for arelatively long period of time, when the actual precipitation takesplace, for the formation of small finely dispersed precipitationsdistributed in the basic material.

Thus, the present invention relates to corrosion protection surfacetreatment of, primarily, so-called high-strength aluminium, whichrelates to a group of heat-treatable alloys, normally containing copper(Cu) and magnesium (Mg) which, by precipitation hardening, are givenhigher strength and mechanical properties. Different alloy compositionsfor producing high-strength aluminium are known to persons skilled inthe art. For example, precipitation hardened alloys containing zinc(Zn), magnesium (Mg) and copper (Cu) as alloy metals, AlZnMgCu alloysbeing numbered among this group.

The corrosion-protecting surface treatment is realised by coating thesurface of the aluminium object with a polymer with improved adhesion-and strength properties and resistance to chemicals. Preferably, polymercompositions containing fluorine are employed. Fluorine polymerssuitable for this purpose are known to persons skilled in the art andneed not be specified further here, but a well-functioning example ofsuch a polymer is polytetrafluouroethylene (PTFE). As adhesive orbinding agent in such PTFR-based polymer compositions, heat-resistantpolymers such as polyphenylene sulphide (PPS) or polyethersulphone (PES)may be employed. At the sintering temperature, for example approx. 400°C., these heat resistant polymers stratify to the metal surface and giveadhesion and hardness.

Hence, according to the method according to the present invention, suchpolymers are melted or sintered fast to the surface of the heatedaluminium object at elevated temperatures in the same heating stage asthe above-mentioned solution treatment, thus avoiding the necessity ofheating the aluminium object in an additional surface coating stageafter the precipitation hardening.

The method according to the invention is applicable to all aluminiumalloys which may be solution treated at such elevated temperatures whichthe polymer composition selected for final use requires for goodsintering and adhesion to the aluminium surface and, vice versa, withall polymer compositions which may afford a good corrosion protectionafter sintering fast at the solution temperature which each respectivealuminium alloy requires.

Each alloy, with its specific composition of alloy components andquantities of them, places different demands on temperatures, stay timesand heating and cooling speeds, respectively. For example, the heatingto the solution temperature may be made in one or several stages duringvarying time intervals, the stay time at the solution temperature may beadapted to the composition and functional requirements of the alloy andthe polymer, respectively, and the cooling speed may be varied withinthe framework of the cooling time and cooling speed dependent propertiesof each respective alloy. The choice of time and temperature for theageing process is also varied in response to the properties of eachrespective alloy. The ageing process normally takes place at one and thesame temperature, buy may also be carried out in one or more stages atdifferent temperatures. For example, a shorter ageing interval at roomtemperature may be carried out, a so-called cold ageing stage, beforeartificial ageing is commenced.

Before the aluminium surface is coated with polymer, it should becleaned and prepared for surface treatment in order to obtain optimumadhesion. This is ideally realised by first heating the aluminiumsurface to elevated temperature such as, for example, approx. 400° C.for burning off organic residues, such as fat and the like, and thensand blasting the surface.

Preferably, the polymer composition is applied in the molten form or inthe form of powder by means of known techniques, such as, for example,thermal spraying (also known as flame spraying), on the surface of thealuminium object before heating to the solution temperature takes place.It is naturally also conceivable to apply the composition in othermanners, such as, for example, in the form of a solution or dispersionwhich is dried and thereafter melted and/or sintered fast on thealuminium surface. Application may also take place during the heatingprocess proper or during the solution time at the solution temperature,with an appropriately adapted process. The polymer coating may beapplied in one or more stages, possibly divided into primer and toplayer, in which event the polymer composition may be varied for thedifferent layers.

The thickness of the polymer composition is adapted to the requirementsof end-use and may, for example, be varied between 10 and 100 μm.

Heating to the solution temperature most appropriately takes place inovens with accurate temperature control, normally air circulation ovens,so-called convection ovens. The heating should take place as rapidly anduniformly as possible in the aluminium material as possible, for whichreason it is appropriate if the oven is pre-heated to the solutiontemperature already when the material object is inserted in place. Theheating time may vary from a few minutes to a couple of hours, dependingon the thickness of the material object and the capacity of the oven.

It is important to carefully follow the temperature limits in solutiontreatment. Too low a temperature will result in poor solution and poorstrength, while too high a temperature may result in discoloration,blister formation or the initiation of melting. The temperature of thematerial is normally held at the solution temperature for approx. 15-60minutes, depending upon the temperature properties of the alloy and thepolymer coating, coarser precipitations which had previously been formedin the material being then dissolved.

In such cases where the alloy requires a longer time or highertemperature than the polymer composition can withstand, the aluminiummaterial may first be partly solution treated, whereafter the polymercomposition is applied so as to be melted/sintered fast during the finalphase of the solution treatment.

The solution-treated aluminium material object must thereafter be cooledso rapidly that no precipitation has time to take place and the alloyadditives remain in an oversaturated solid solution, which isprecondition for the final strength of the material to be sufficientlyhigh. Certain alloys are considerably more sensitive for sufficientcooling speed than others, in order to achieve maximum strength afterthe precipitation hardening. For example, alloys of the 7075 type, whichhave a very demanding dependence on cooling time and cooling speed,require a cooling speed of at least 300° C./s. Cooling normally takesplace in water, but may also be put into effect using water spraying orair cooling, among other things depending on the thickness of thematerial. What is crucial is that the cooling takes place rapidly andthat the temperature of the coolant is maintained more or less constant.

The ageing stage is thereafter carried out by storage at roomtemperature (cold ageing) or at elevated temperature (artificialageing). The alloy atoms which are in oversaturated solution in thematerial after the solution treatment form, by diffusion, minorprecipitations which increase the strength of the material. Thus, theageing process takes place already at room temperature, but is slow. Inorder to entirely inhibit the ageing process in the material for a shorttime, it may be stored at a temperature lower than −15° C. Ageing atelevated temperature generally gives a sufficiently fine precipitationdistribution in a reasonable time and, as a rule, gives maximumstrength. Further improved strength may be obtained by causing thematerial to cold age a short time before the artificial ageing. Itgenerally applies in this context that a higher ageing temperaturepermits a shorter ageing time, but with a certain loss of strength. Somealloys age sufficiently over a reasonable time (a few days) at roomtemperature, while other alloys are always artificially aged. Theabove-mentioned 7075 alloys are artificially aged, for example, often atapprox. 120° C. Artificial ageing temperatures normally vary between 100and 200° C., while artificial ageing times normally vary between 5 and48 hours. Longer times and higher temperatures generally result inlarger, but fewer precipitation particles. Thus, it is a matter ofoptimising the ageing cycle and thereby the size and distribution of theprecipitations for each respective aluminium alloy so that an optimumbalance of the properties of the material is achieved. Maximum tensilestrength must, as a rule, be set off against a certain loss of, forexample, corrosion resistance. The degree of hardening in artificiallyaged alloys is disclosed by T-designations, such as, for example, T5 toT10. Hardening degrees T6 and T7 are given for materials which, aftersolution treatment and cooling, have been treated with precipitationartificial ageing. T6 hardened aluminium material has, as a rule, thehighest possible strength practically without losing any other keyproperties. T7 material is so-called “over-aged” at generally higherartificial ageing temperatures as compared with T6 material of the samealloy, which permits higher dimensional stability in use at highertemperatures in, for example, engine parts.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

The present invention will now be described in greater detail in oneconcrete embodiment, with reference to accompanying FIG. 1 whichschematically illustrates a precipitation hardening cycle for onepreferred embodiment of the method according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENT

As starting material, use is made of an AlZnMgCu alloy designatedAA7075, for the production of a high-strength aluminium material object.The material is, for example, intended for machine parts in the sealingunit of a packing and filling machine of the above-described type. Inorder to meet the requirements of strength, precipitation hardening forhardening to T6 or T7 is to be carried out. The processed and formedaluminium material object should first be prepared for surface treatmentby means of adhesion-promoting measures, for example suitably by firstheating the aluminium surface to approx. 400° C. for burning off fatmolecules and other organic residues (a) and subsequently sand blastingthe surface (b). The precipitation hardening process proper issubsequently carried out in accordance with the present invention inconnection with surface coating and sintering of a protective layer of aPTFE-based composition with a high melting point, such as, for example,“Accolan Silver”® from the “Accoat Group”.

The polymer is applied on the aluminium material object prepared forprecipitation hardening at room temperature (c), by means of thermalspray coating, i.e. by melting granules or powder of the polymercomposition, for example with a flame, and spray-applying the moltenmaterial on the surface of the aluminium material. The polymer isapplied to a suitable thickness of approx. 10-120 μm, preferably approx.20-60 μm and most preferably approx. 40 μm.

The polymer-coated aluminium object is thereafter heated to solutiontemperature during a relatively short time (d). The heating cycle maypossibly take place in two or more stages (e) so as to avoid blisterformation in the polymer layer. When the material has reached atemperature of approx. 420° C., it is kept at this temperature for astay time of approx. 15 minutes (f). It is essential for the finalproperties of the material that aluminium alloys of the 7075 type areheated for solution treatment to approx. 420° C. (at least 415° C.), see“ASM Specialty Handbook—Aluminum and Aluminum Alloys”, pp. 300-301, FIG.6. During the stay time, the alloy atoms are dissolved in the aluminiummaterial at the same time as the polymer is melted/sintered fast on thesurface of the aluminium object.

At the end of the stay time, the object is rapidly cooled to roomtemperature with water or air, preferably water (g). The coolingoperation takes place at a speed of at least 300° C./s, and the objectis then retained in cooling water for approx. 60 minutes (h).

Before the artificial ageing stage is commenced, the object may possiblybe allowed to cold age during a brief period of time at room temperaturein air for approx. 150 minutes (i), higher final strength being thusobtained. Extremely high strength will, however, be obtained even if theabove-mentioned cold ageing in air is dispensed with.

Finally, the aluminium material object is artificially aged preferablyat at least approx. 150° C. for approx. 24 hours for the finalprecipitation hardening (g), whereby the hardening degree T7 isachieved. An artificial ageing temperature of approx. 120° C. alsofunctions well and possibly provides a harder material but with lowerresistance to stress corrosion (hardening degree T6). Artificial ageingat approx. 150° C. realises a material with satisfactory hardness forthe above-mentioned specific practical application and good resistanceto stress corrosion.

The above-described, specifically selected alloy compositions andcoating polymers merely constitute examples among many other conceivablealternatives, and it will be obvious to a person skilled in the art thatnumerous modifications and variations may be put into effect withoutdeparting from the inventive concept of the method according to thepresent invention as this is defined in the appended claims. Alloys areadapted and precipitation hardened using technologies known to personsskilled in the art, taking into account the requirements placed on thematerial in use.

As will have been apparent from the foregoing description, the presentinvention thus realises a novel method of surface treating, by polymercoating, and improving the corrosion resistance in objects ofhigh-strength aluminium and, at the same time, maintaining the superiormechanical properties and high strength of the material.

What is claimed is:
 1. A method of coating and precipitation hardeningan aluminium alloy object, the method comprising: providing at least onelayer of a protective polymer material which comprises afluorine-containing polymer on a surface of the aluminium alloy objectthereby forming a coated aluminium alloy object; and heating the coatedaluminium alloy object to an elevated temperature of at leastapproximately 420° C., causing constituents of the aluminium alloy tomerge into a solution and causing the polymer material to sinter or meltfast to the surface of the aluminium alloy object thereby forming astrongly-adhered, permanent corrosion resistant coating.
 2. The methodas claimed in claim 1, wherein the aluminium object is coated withpolymer before being heated to the temperature solution treatment. 3.The method as claimed in claim 1, wherein the aluminium object is coatedwith polymer while being, or after having been heated, to the elevatedtemperature.
 4. The method as claimed in claim 1, wherein the polymer iscoated in two or more layers.
 5. The method as claimed in claim 1,wherein the polymer substantially comprises PTFE.
 6. The method asclaimed in claim 1, wherein the polymer composition is coated by meansof thermal spraying.
 7. The method as claimed in claim 1, wherein thealuminium object is heated in two stages to reach the elevatedtemperature.
 8. The method as claimed in claim 1, wherein the polymercoating is sintered or melted fast on the aluminium object during a staytime at the elevated temperature of approximately 15 minutes.
 9. Themethod as claimed in claim 1, wherein the aluminium object, aftersolution treatment at the elevated temperature, is rapidly cooled toroom temperature and thereafter precipitation hardened by means ofartificial ageing at approx. 150° C. for approx. 24 hours.
 10. Themethod as claimed in claim 9, wherein the aluminium object, prior toartificial ageing, is aged at room temperature for approx. 150 minutes.11. The method as claimed in claim 1, further comprising: cooling thecoated aluminium object from the elevated temperature; and ageing thecoated aluminium alloy object.
 12. The method of claim 1, wherein thepolymer material has a composition such that, upon sintering, thepolymer stratifies to the surface of the aluminium alloy object, therebypromoting adhesion and hardness.
 13. An aluminium object coated withpolymer and surface treated, produced by means of the method as claimedin claim 1.